Precious metal containing ores treated for recovery of gold and other precious metals are increasingly of the so-called refractory type in which a sulphide mineral contains freely, or in association, a precious metals constituent. The sulphide mineral, often of the pyrite or iron sulphide variety-though other base metal sulphides may be present, may then occur in association with a gangue mineral. In addition to these components the ore may also contain species such as tellurides.
By "refractory" is meant that the ore is intractable to treatment by direct cyanide or other leaching processes. The intractability may vary in degree thus ores may be said to be partially refractory. In the most severe cases, no economically significant degree of gold recovery is achieved and the ore is said to be fully refractory. For example, where ore grains consist mainly of pyrite encapsulated gold, the ore may be especially difficult to treat by direct leaching and may be fully refractory.
Lanyon, M. and Floyd, J M, "Recovery of Gold from Refractory Ores and Concentrates using the Sirosmelt Reactor", Research and Development in Extractive Metallurgy AusIMM Conference, Adelaide 1987 discloses the difficulties in treating such ores. For example, if the sulphide mineral is, for example, pyrite or another iron sulphide mineral direct cyanidation becomes expensive to the point of becoming uneconomic.
Therefore, in an effort to avoid the need for roasting plant followed by hydrometallurgical recovery of gold, Floyd proposes a direct smelting process for the treatment of such ores in which a matte phase containing the precious metals component is formed in a Sirosmelt reactor. From the low grade matte, the precious metals may be recovered by use of a collector such as copper, lead or iron.
Pyrometallurgical routes are unlikely to be economic because of the very large volume of material that must be treated due to the low grade of the ore.
Therefore, a technique is required that will enable the precious metals component to be recovered in as inexpensive and efficient a manner as possible and this mandates the recovery of the usually sulphide or telluride component by some means of concentration.
Bacterial leaching techniques may be employed to solubilise a sulphide mineral, thereby liberating the precious metals component for recovery of the precious metals by cementation or other electrochemical techniques. However, rate of solubilisation, high temperatures and lack of water may make this an unsuitable technique in many situations.
Flotation is used in the treatment of refractory/partially refractory auriferous material to separate gangue minerals and produce a significantly smaller volume of concentrate for further processing. The overall gold recovery is most dependent upon the first stage process recovery; in this case, flotation. Therefore, the degree of recovery from flotation and grade of the flotation concentrate are desired to be as high as economically and technically feasible in order to ensure viability of material treatment for gold and other precious metals recovery.